1. Field of the Invention
This invention relates to photomultiplier tubes and more particularly to such a tube with improved means for controlling the gain thereof.
2. Description of the Prior Art
A photomultiplier tube (PMT) is one of the most, if not the most, sensitive optical detector for operating in the visible and ultraviolet spectrum. A complete description of the theory, design and application of the photomultiplier tube is given in Photomultiplier Handbook published by RCA Corporation (PMT-62, 1980). In certain applications with widely varying background illumination, however, it is necessary to vary the gain to remain within the anode current rating of the PMT. The conventional approach to achieve this result is to reduce the bias voltage of the entire dynode resistive divider. The reduced voltage is reflected as a reduced voltage on each stage of the chain and the overall gain is thereby reduced.
The effect of this conventional approach is that reduction of gain of the first dynode stage adjacent to the cathode tends to produce a degraded noise figure for the PMT because noise associated with subsequent stage contributes more significantly to the total noise. While the gain of the first stage may be kept constant by use of a voltage regulator diode, such as a Zener diode, the diode itself often introduces undesirable noise. Another disadvantage is that a wide swing in gain requires a wide voltage swing. Typically, a conventional PMT may require a 700 volt swing to effect a 3 decade gain change.
This invention is directed toward dynode bias circuit which overcomes these disadvantages.